The art of digital communications has been plagued for a long time by the incompatibility of the two main strategies for moving large amounts of data in the form of binary digital signals from one device to another in a very short period of time.
The first of the two strategies, baseband transmission, involves sending a controlled physical manifestation (e.g. a digital signal) from one device through a carrying medium, such as a wire or cable, to another device having some means for noticing the physical manifestation and sensing and interpreting the nature of the control. Typically the physical manifestation is a particular voltage level, controlled as to level and duration, carried over an electrical wire to some kind of sensing device at another point on the wire where some means is employed for noting the voltage levels and their durations, which when decoded reveal a message. Given the elegant simplicity of the baseband system it is not surprising that very great data transmission speeds can be obtained with it.
A feature of baseband systems which contributes to their speed is the abruptness of the typical baseband signal. For example, a transition from a high voltage to a low voltage in a typical baseband signal is accomplished in less than 10 nanoseconds, whereupon, the data represented by that transition is immediately ready for use in a receiving device.
An important aspect of communication network systems concerns how to deal with simultaneous transmission of messages on the same medium. "Rules of good behavior" are typically incorporated into a communication system to deal with situations where multiple messages may collide. Means are typically provided in communications device, therefore, to sense when access to the line is open, and further to sense whether a second signal is being put on the line to collide when the first signal is transmitting, whereupon the first signal is aborted and retransmitted at a later time. These means are called "Carrier Sense Multiple Access" or CSMA procedures, and when they also have the "Collision Detection" feature, they are called CSMA/CD procedures. These rules of behavior are implemented on many systems, baseband and broadband alike.
The second of the two strategies, broadband transmission, differs from baseband in a fundamental way: where baseband contemplates a digital signal placed directly on the wires, the broadband signal is represented by a modulation, a carefully controlled change of the carrier wave, such as in amplitude or frequency. By careful attention to selection of an appropriate portion of the electromagnetic spectrum in which to transmit the carrier wave, it is possible to provide a broadband medium where many discrete channels, or bands, may be recognized, each of which can carry its own set of messages simultaneously with those of each other channel, or band. Thus, by the provision of a large plurality of message bands, a very large amount of data may be carried over a broadband system.
Before data in the form of a series of modulations on a carrier wave can be used by a device to which it has been sent, it must first be recognized and then translated into a form usable by the device. This process is called demodulation, and is done by a device called a "modem". The word "modem" comes from the two reciprocal processes which it performs: modulating data and demodulating it. A broadband modem must also have means for selecting the particular frequency or frequencies of the carrier wave from which it is desired to reads the incoming data, or upon which it is desired to send outgoing data.
In summary, baseband system are single-channel and digital while broadband systems are multichannel, analog and use a carrier wave.
In data communications, a packet is a group of binary digits, including data and control elements, which is switched and transmitted as a composite whole. The data and control elements and possibly error control information are arranged in a specified format. Regardless of the type of communications medium, devices receiving packets must by some method be able to detect when the end of packet has occurred. Baseband systems normally use no special indicator, but rather simply assume that when they stop receiving data the packet is ended. This is possible because they are really equivalent to a piece of wire carrying digital data, and as soon as the transmitting device stops sending data, the receiving device can detect the lack of data transfer.
A major difficulty in the way of easily interfacing broadband packets into a baseband system is the fact that the broadband carrier wave does not die as abruptly after transmission of a data packet, as does the baseband signal. The end of packet is detected in a baseband system by lack of data transfer on the medium. However, since the broadband carrier does not immediately die off, the broadband end-of-packet is not reliably recognized using the baseband method. Instead, the modem continues to detect data on the medium where none should actually exist. These extra binary digits of data are called "dribble" bits.
One system which operates in near real-time works by requiring a "post-amble" of a peculiarly ordered set of bits at the end of the data packet after the regular end-of-packet code. The ordered set of bits are calculated uniquely for each packet. The post-amble ending code is compared to a known ending code. As the packet passes through a shift register in the receiving device. The end of the packet is thereby recognized when the post-amble ending code matches the known ending code. A drawback of the post-amble system is that the post-amble adds extra overhead to the transmitting-receiving process, which adds to the complexity and expense of the data handling.
There is thus a need for an interface device, operative in real time, for easily and economically moving data between two incompatible communications systems such as broadband and baseband. The present invention is directed toward filling that need.